CHAPTER 4:HEAT
4.1
Understanding the thermal equilibrium
Define:
| a)Temperature | The measure of the degree of hotness of an object. SI Unit Kelvin K A hot object is at higher temperature than a cold object. |
| b)Heat | Form of energy, measured in Joules J. Heat is transferred from hotter object(higher temperature)to colder object(lower temperature) When an object is heated, it will absorb heat energy and the temperature will increase. When an object is cooled, it will release heat energy and the temperature will decrease |
| c)Thermal contact | Two object are in thermal contact when heat energy can be transferred between them |
| d)Heat transfer | When two object with different degrees of hotness come into thermal contact, heat energy is transferred between the two object |
| e)Mechanism of thermal equilibrium | Energy is transferred at a faster rate from the hotter object to the colder object. Energy also transferred from the colder object to the hotter but at a slower rate There is a net flow from the hotter to the colder object The hotter object cools down while the colder object warm up. After some time, energy is transferred at the same rate between the two object. There is no net heat transfer between the objects. The two objects are said to be in thermal equilibrium. |
Thermal equilibrium
When the two object are in thermal equilibrium, there is no net flow of heat between them.
Two object in thermal equilibrium have the same temperature.
Example of thermal equilibrium
A hot drink can be cooled by adding a few ice cubes to the drinks. Heat from the hot drinks is transferred to the colder ice until thermal equilibrium between the ice and water is reached. The final temperature of the drink equals the final temperature of ice.
UNDERSTANDING SPECIFIC HEAT CAPACITY
Shown here is the thermal motion of a segment of protein alpha helix. Molecules have various internal vibration and rotational degrees of freedom. This is because molecules are complex objects; they are a population of atoms that can move about within a molecule in different ways. This makes molecules distinct from the noble gases such as helium and argon, which are mono-atomic (consisting of individual atoms). Heat energy is stored in molecules’ internal motions which gives them an internal temperature. Even though these motions are called “internal,” the external portions of molecules still move—rather like the jiggling of a water balloon.
Application of Specific Heat capacity
Specific Latent Heat
The specific latent heat of a substance is the energy which is required to change 1 Kg of a substance from a certain physical condition to another physical condition without any change in temperature.
The heat energy absorbed by the ice does not cause the increase in temperature. The energy absorbed is not transferred to the molecules of ice as kinetic energy.
The heat absorbed is known as latent heat.
Heat energy needs to be supplied to change a substance from solid to liquid phase and from liquid to gaseous phase.When a solid melts, heat is absorbed but the temperature remains constant.
When a a liquid is boiling, heat is also absorbed but the temperature remains constant.
From the principle of conservation of energy, latent heat must be given out when a gas condenses to become a liquid and when the liquid solidifies to the solid phase. These two processes also occur at constant temperature.
The four main changes of phase are melting, boiling, condensation and solidification.
Two object in thermal equilibrium have the same temperature.
A hot drink can be cooled by adding a few ice cubes to the drinks. Heat from the hot drinks is transferred to the colder ice until thermal equilibrium between the ice and water is reached. The final temperature of the drink equals the final temperature of ice.
UNDERSTANDING SPECIFIC HEAT CAPACITY
Heat capacity,C | The unit for heat capacity is J°C |
Specific heat capacity,c | The unit of specific heat capacity is Jkg-1°C-1. Therefore, when a body of a mass m and specific heat capacity, c, absorbs a quantity of Heat, Q, then its heat will increase by θ. Therefore Q = mc θ |
The physical meaning of specific heat capacity,c. | When two objects of equal mass are left to cool down,the temperature of the object with smaller heat capacity will drop faster. |
A substances with a small value of specific heat capacity | For example,metal like iron,steel,copper and aluminium is used as pots and pans because they can be quickly heated up when there is only small heat absorption. Sensitive to temperature changes.A thermometer has low specific heat capacity so it enables heat to be easily absorbed and released even when small quantities of heat are involved. |
A substances with a high value of specific heat capacity | Require more heat to raise its temperature by a a specific amount. Can absorb a great amount of heat without a high in temperature. |
Shown here is the thermal motion of a segment of protein alpha helix. Molecules have various internal vibration and rotational degrees of freedom. This is because molecules are complex objects; they are a population of atoms that can move about within a molecule in different ways. This makes molecules distinct from the noble gases such as helium and argon, which are mono-atomic (consisting of individual atoms). Heat energy is stored in molecules’ internal motions which gives them an internal temperature. Even though these motions are called “internal,” the external portions of molecules still move—rather like the jiggling of a water balloon.
Cooking pot a)Copper base b)Wooden handle | Low specific heat capacity.The pot become hot very quickly.This enables quick cooking of the food in the pot.High density.The heavier base ensures that the pot is stable and will not topple over easily Large specific heat capacity.The handle will not become too hot when heat is absorbed.Poor conductor of heat. |
Sea breeze | Land has a smaller specific heat capacity than sea.Faster increase in temperature,land is warmer than the sea. Air above the land is heated up and rises Cooler air from the sea moves from toward the land as the sea breeze. |
Land breeze | At night,heat is lost from the land and the sea. Sea has a larger specific heat capacity so sea is warmer than the land. Cooler air from the land moves towards the sea as land breeze |
The image of land and sea breeze
Specific Latent Heat
The specific latent heat of a substance is the energy which is required to change 1 Kg of a substance from a certain physical condition to another physical condition without any change in temperature.
The heat energy absorbed by the ice does not cause the increase in temperature. The energy absorbed is not transferred to the molecules of ice as kinetic energy.
The heat absorbed is known as latent heat.
Heat energy needs to be supplied to change a substance from solid to liquid phase and from liquid to gaseous phase.When a solid melts, heat is absorbed but the temperature remains constant.
When a a liquid is boiling, heat is also absorbed but the temperature remains constant.
From the principle of conservation of energy, latent heat must be given out when a gas condenses to become a liquid and when the liquid solidifies to the solid phase. These two processes also occur at constant temperature.
The four main changes of phase are melting, boiling, condensation and solidification.
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